Timing and control and data acquisition for a multi transducer ground penetrating radar system

ABSTRACT

An improved multi transducer for a ground penetrating radar system (GPR) having a complete circuit for internal timing of signal emission, detection, digitalization and recording of data.

FIELD OF THE INVENTION

[0001] This invention relates in general to ground penetrating radarsystem, and more particularly to a multi transducer for a groundpenetrating radar system (GPR).

BACKGROUND OF THE INVENTION

[0002] There is a growing demand for GPR systems that have the abilityto acquire data with more than one transducer system. The ability to runmore than one transducer system at a time is extremely complex given thenature of the problem. Complex control and detailed accurate timingdesign are needed in the system.

[0003] In current practice, systems have one transmitter and onereceiver unit. Generally GPR systems obtain data along a measurementtraverse with the transmitter and receiver in a fixed geometricalconfiguration with respect to one another (prior art, FIG. 1); the GPRsystem as a whole is moved over the ground or medium to be explored(Annan, A. P., Davis, J. L., Ground Penetrating Radar—Coming of Age atLast, 1997; Proceedings of the Fourth Decennial International Conferenceon Mineral Exploration (Exploration'97), Toronto Canada, Sep. 14 to Sep.18, 1997).

[0004] References to the utilization of more than one transmitter orreceiver are limited. Prior attempts have been made as described in U.S.Pat. No. 5,248,975 issued to Schutz, A. E., entitled “Ground ProbingRadar with Multiple Antenna Capability”.

[0005] There are four major problems that have to be overcome. The firstproblem is that the acquisition of ground penetrating radar traces insingle transient waveform capture process, in digital form (or evenanalog form) is virtually impossible. Current commercially availableanalog to digital (A/D) converters are simply not fast enough nor dothey have sufficient dynamic range to record the signals required formany of the GPR applications.

[0006] As a result, GPR systems resort to some sort of repetitive signalin order to capture the desired data. The most common approach is to useequivalent time sampling. Other approaches are to use a step frequencyCW that acquires data in the frequency domain by detecting the in-phaseand quadrature response of the transfer function; the time domain signalis created by fourier transform.

[0007] A third approach is to use a fast A/D converter with few bits(i.e. limited dynamic range) and then stack the resultant signal formany, repetitions in order that the resolution can be brought up. Afourth approach is to transmit some stream of random signals and use acorrelation technique to extract the impulse response. As a result,considerable time is needed at each observation point to acquire data ofa satisfactory nature. The integration of this complex, individualsignal capture process into the overall acquisition timing andsynchronization of events with multiple units is a complicated task.

[0008] The second major problem in trying to operate more than 2 unitsis that 2 transmitting signals operating at the same time can interferewith one another. If one wants to have two units, which are collectingindependent information but operating at the same time then it isimportant that the signals from the transmitters do not get emitted atexactly the same time so that the two data sets can be acquired withhigh fidelity. In some instances it is desirable to have thetransmitters operating simultaneously, but in this case one wants tomake sure that the timing of the transmitters is perfectly synchronizedin order to enhance signal from the ground.

[0009] The third problem is that the antennas which are the transducerscreate, radiate and detect the electromagnetic signals which aretransmitted into the ground are highly dependent on the surroundings.When multiple transducers are placed in close proximity to one another,the transducers can interact in an unpredictable fashion and generatespurious signals.

[0010] The final problem is with the spatial distribution of thetransducers. Since the signals that are being measured are radio wavesthat travel at the speed of light, all of the times involved in themeasurement process are very short. Since the subsurface spatialdimensions may be similar to the separation distances between GPRcomponents, the travel times on the inter connecting cabling of thesystems can become as large if not larger than the travel times of thesignals through the media being probed. As a result, it is importantthat any timing system be able to recognize these time differences andadjust times to eliminate the time delays associated with spatialdistribution of the transducers.

[0011] FIGS. 2-5, show the most commonly envisaged multi-unit systems.FIG. 2 shows the use of multi transducer systems where the objective isto obtain data records from a variety of separations between thetransducers. Many applications could benefit if data from a multiplicityof separation could be acquired simultaneously. Fisher, E., McMechan, G.A., and Annan, A. P., Acquisition and Processing of Wide-Aperture GroundPenetrating Radar Data; 1992; Geophysics, Vol. 57, p. 495-504, andGreaves, R. J. and Toksoz, M. N, Applications of Multi-Offset GroundPenetrating Radar; Proceedings of the Symposium on the Application ofGeophysics to Engineering and Environmental Problems, 1994; (SAGEEP'94),p. 775-793 discuss the use of variable offset measurements and theenhancement of the data that can be achieved by coherent spatialstacking in the spatial dimension.

[0012] In some applications, the making of multiple separationmeasurements made at each station along the transect line, is calledmulti-fold surveying. Multi-offset data available at every measurementpoint allows for the extraction of a velocity cross-section, anattenuation cross-section and an enhancement of data by determining anoptimum spatial stacking velocity structure.

[0013] The second type of multi-channel system is depicted in FIG. 3. Inthis case the objective is to try and cover a larger area more quickly.Many GPR applications require acquisition of data on a series ofparallel lines in order that a large area can be covered to obtain athree dimensional volume view of the ground.

[0014] One way of improving such surveys is to have a number ofmeasurement systems mounted side-by-side and have these transported overthe ground simultaneously. In FIG. 3, a one channel system is shownsequentially measuring up and down 4 lines to acquire the same data that4 transducers traversing once simultaneously over the four lines wouldachieve as shown in FIG. 3b. It is useful to note in this applicationthat the individual units can more or less operate independently. Theydo not really require synchronous sampling times but it is desirable ifthe transmitters be set up to operate at different staggered times toeliminate any potential of interference between the units caused bysimultaneous operation of the individual units.

[0015]FIG. 4 depicts still another type of application where multipletransducers or measurements are desirable. Quite often the bandwidth ofGPR systems is limited by the intrinsic characteristics of antennas. Fordetailed study of the subsurface, a number of systems with differentfrequency bandwidths and corresponding different physical sizes may haveto be traversed along the same line in order to achieve full coverage ofthe subsurface.

[0016] At present, this type of operation is achieved by surveying theline a number of times as depicted in FIG. 4, once with each transducer.The whole operation could be speeded up if all (three transducers in theexample shown) transducers could be moved simultaneously down the lineat one time and the same data acquired.

[0017] The most general use of multi-unit systems is depicted in FIG. 5and consists of a full array of transmitters and receivers spread overan area. The operation of transmitters either independently orsynchronously together in time, as well as all of the receiversoperating and acquiring data synchronized in time, provides a powerfulmeans of subsurface imaging. The whole package shown could betransported along the line to provide multi-offset continuous data in athree dimensional fashion. Such data acquisition then lends itself touse of synthetic aperture processing or the equivalent multifold 3Dseismic processing concepts that are commonly applied in the petroleumindustry. Such an application requires synchronization of all of thetiming in the units that are spatially distributed.

[0018] Equivalent time sampling (ETS) is a means of using multiplerepetitions of a transient signal to capture a transient waveform(Mulvey, John, Sampling Oscilloscope Circuits; 1970; InternalPublication of Tektronix, Inc., Beaverten, Oreg.). Other modes ofoperations such as continuous wave, step frequency or instantaneouscapture and stacking can use some of the concepts outlined here.

[0019] As indicated previously, ETS receivers require successiverepetitions of the signal waveform to be recorded in order that it canbe acquired. Fisher (supra) provides information on ETS and some of thetypes of systems that have evolved.

[0020] Analog ETS systems were spawned in the 1960's and 1970's. FIG. 6depicts a typical ETS. A timing circuit is required which will provide avery controlled time delay between when a signal is created and the timeat which a measure of the waveform sampled over a short time interval isacquired. Historically two analog ramps, one slow and one fast, wereused to drive a comparator that would provide a time delayed triggeroutput.

[0021] For the ETS shown in FIG. 6, the key feature is that the receivetrigger is delayed in time progressively on every repetition of thetransmit pulse. This time that is dictated by a control clock delay,increases from a minimum value to a maximum value over a fixed amount oftime (i.e., N repetitions of the control clock). When the number ofdesired repetitions of the control clock has been reached, the wholesystem is reset and it starts over again. To work properly the controlclock has to be very stable and regular.

[0022] Using a sample and hold or a sampling head circuit, the transientsignal is captured over a short interval in time is output from thesampling device as a continuous analog voltage. Provided the controlclock is stable and delay time varies linearly, the analog voltage is areplica of the transient waveform input but which is slowed down intime. Time stretching of 1,000:1 or even 1,000,000:1 is common.

[0023] The captured signal in the case shown in FIG. 6 requires Nrepetitions of the master clock and the transmitted signal to acquireone replica of real signal. The real time transient waveform will besampled over a real time interval NΔt where Δt is the amount thereceiver trigger is delayed on each successive cycle of the system. Whatcharacterizes such a system is the repetition rate. This is the clockshown in the schematic diagram in FIG. 6. If the repetition period ofthe clock is P, then the real time signal interval NΔt will be acquiredin an ellipsed time of NP. This is an equivalent time stretch factorthat is determined by the ratio $\frac{P}{\Delta \quad t}.$

[0024] When using analog oscilloscope displays or audio tape recordersfor data acquisition, the analog signal is stretched to the audiofrequency range from the radio frequency range. This enables datadisplay recording and replay using lower-cost and lower speedelectronics.

[0025] The basic analog ETS system as depicted can be used to supportmultiple transmitters or receivers. If the triggering signals can besequenced by a computer, or some sort of preprogrammed logic array, thena number of channels of data can be acquired as shown in FIG. 7.

[0026] In this situation the receiver and transmitter triggers as shownin FIG. 6 are fed through a switching network which enables transmitteror receiver units to be switched or enabled or disabled. The output ofthe receivers are analog traces which can then be digitized or displayedon an oscilloscope or recorded on an analog tape (Mulvey, John, supra).

[0027] There are drawbacks in this approach. If there are M transmitterand receiver pairs to be switched, then the acquisition time increasesto M×NP. In other words, data acquisition rate is slowed down. If asingle transmitter and a multiple set of receivers are to be used toacquire time synchronous data, then the full waveform recording sequencefor the receivers must be required before switching to anothertransmitter and repeating the sequence. Such multiplexing reduces therate at which the system can be moved. the real time signal interval NΔtwill be acquired in an ellipsed time of NP. This is an equivalent timestretch factor that is determined by the ratio$\frac{P}{\Delta \quad t}.$

[0028] When using analog oscilloscope displays or audio tape recordersfor data acquisition, the analog signal is stretched to the audiofrequency range from the radio frequency range. This enables datadisplay recording and replay using lower-cost and lower speedelectronics.

[0029] The basic analog ETS system as depicted can be used to supportmultiple transmitters or receivers. If the triggering signals can besequenced by a computer, or some sort of preprogrammed logic array, thena number of channels of data can be acquired as shown in FIG. 7.

[0030] In this situation the receiver and transmitter triggers as shownin FIG. 6 are fed through a switching network which enables transmitteror receiver units to be switched or enabled or disabled. The output ofthe receivers are analog traces which can then be digitized or displayedon an oscilloscope or recorded on an analog tape (Mulvey, John, supra).

[0031] There are drawbacks in this approach. If there are M transmitterand receiver pairs to be switched, then the acquisition time increasesto M×NP. In other words, data acquisition rate is slowed down. If asingle transmitter and a multiple set of receivers are to be used toacquire time synchronous data, then the full waveform recording sequencefor the receivers must be required before switching to anothertransmitter and repeating the sequence. Such multiplexing reduces therate at which the system can be moved.

[0032] There is no simple way in which the timing associated with delaysalong the interconnect lines can be handled in any systematic fashion.This may be developed into the system by calibrated cables or may behandled in post acquisition but it is not readily accommodated by theanalog ETS configuration shown.

[0033] Therefore a multi transducer ground penetrating radar system in acompact self-contained GPR unit is desirable.

SUMMARY OF THE INVENTION

[0034] An object of one aspect of the present invention is to provide animproved multi transducer for a ground penetrating radar system.

[0035] In accordance with one aspect of the present invention, there isprovided a more enhanced digital equivalent time sampling approach.

[0036] In accordance with another aspect of the present invention, themulti transducer for a ground penetrating system allows for a completelyoperational, self-contained system. The present invention may containcomplete compact circuits for internal timing of signal emission,detection, digitalization and recording of data.

[0037] Conveniently, the present invention allows for total independenceand the ability to pass data to a common or several independentacquisition and display systems. Preferably such an operation is bestwhen there is a minimal signal coupling between devices.

[0038] In accordance with another aspect of the present invention themulti transducer ground penetrating system may be commanded to acquiredata in an interleaved fashion but operate in a totally self containedmanner by a master computer or clock. This mode of operation is optimalwhen there is signal coupling between the invention/devices but the dataare to be treated as independent data streams.

[0039] Conveniently the time bases of the present invention can besynchronized such that all the devices can detect and record signalsfrom all other devices. Operation in this manner is beneficial forenhancement and extraction of information contained in the spatialplacement of the invention. The ability to process all signalscoherently allows for the implementation of real time or postacquisition synthetic aperture and multifold signal processing such asused in the petroleum seismic.

DETAILED DESCRIPTION OF THE DRAWINGS

[0040] A detailed description of the preferred embodiment is providedherein below by way of example only with reference to the followingdrawings, in which:

[0041]FIGS. 1a and 1 b are schematic representations of the measurementand response of a ground penetrating radar system.

[0042]FIGS. 2a, b, c. are schematic representations of variations ofmulti offset measurements.

[0043]FIGS. 3a and 3 b are schematic representations of mapping an areain 3D volume.

[0044]FIGS. 4a and 4 b are schematic representations of the use ofdifferent frequency ground penetrating radar systems.

[0045]FIG. 5 is a diagram of the preferred configuration of transmittersand receivers for a ground penetrating radar system.

[0046]FIGS. 6a and 6 b are schematic representations of a conventionalanalog equivalent time sampling system.

[0047]FIG. 7 is a schematic representation of an analog time-basedequivalent time sampling system using multiple transducers andreceivers.

[0048]FIGS. 8a and 8 b are schematic representations of a digitalequivalent time sampling system.

[0049]FIG. 9 is a schematic representation of a multi digital equivalenttime sampling system circuitry.

[0050]FIG. 10 is a schematic representation of a multi digitalequivalent time sampling system including the clock, sampler, andmicroprocessor.

[0051]FIG. 11 is a schematic representation of the triggering circuit ofthe multi digital equivalent time sampling system.

[0052]FIGS. 12a, b, c, d are schematic representations of the multidigital equivalent time sampling system.

[0053]FIG. 13 is a table outlining the different applications of theinvention.

[0054] In the drawings, preferred embodiments of the invention areillustrated by way of example. It is to be expressly understood that thedescription and drawings are only for the purpose of illustration and asan aid to understanding and are not intended as a definition of thelimits of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0055] In the description that follows, like parts are marked throughoutthe specification and the drawings with the same respective referencenumerals. The drawings are not necessarily to scale and in someinstances proportions may have been exaggerated in order to more clearlydepict certain features of the invention.

[0056] Digital equivalent time sampling (DETS) is a modern approach toETS. The basic concept is depicted in FIG. 8. With DETS, a single sampleof a transient waveform is acquired at a time delay D after stimulationof the response. In DETS, the time delay D is discretized andprogrammable. In initial DETS systems, the time delay was defined as nQwhere n is an integer and Q is a fixed time interval. The result is

D=nQ 0<n<N  (1)

[0057] In general the value of N was some finite limit dictated by thedigital logic of the embodiment by a microprocessor. Typically themaximum n values would 2¹⁰ and 2¹² in early devices. This type ofdigital equivalent time sampling is common in commercial products.

[0058] In this case a more sophisticated clocking sequence is used togenerate delays. In this case time delays have the form

D=mP+mQ  (2)

[0059] where N×Q=P which gives a coarse and fine delay capability to thetiming. The reason for this extended approach is that for generalpurpose application in multi channel DETS, the time delays which may berequired are much larger than a single DETS requirement designed systemcan accommodate. By approaching the construction this way one maintainsthe fine scale Q resolution but at the same time extends the offsetrange to a much larger range.

[0060] DETS provides a tremendous versatility in the equivalent timesampling concepts. The most important aspect is that there is no longera need for a repetitive clock rate which dictates the systems outputdata rate. Each point sampled using a DETS system is an acquisition thatis totally independent of any other point acquired.

[0061] For example the system could gather sample points once per secondfor a while, then once every 10 seconds or once every millisecond andthe resulting waveform would be captured with equal validity as if allof the samples were acquired at a one microsecond interval. In otherwords, sampling is now an individual point event and no longer requiresa clock or a fixed repetition rate for the transmitter or any other partof the circuit. In a DETS, there is usually a stable crystal clock whichis used to provide the fundamental period P and the fine samplinginterval Q but this clock has no relationship to the rate at whichsamples are acquired.

[0062] With a DETS base system, the equivalent to the analog traceacquisition discussed in FIG. 6 can be emulated by having a computer orsome programmable or hardwired logic develop a series of time delays D₁

D₁=Δt

D ₂=2Δt

D ₃=3Δt  (3)

[0063] where Δt in this case equals pQ where p is an integer (i.e., Δtmust be an integer multiple of the fundamental programmable delayinterval Q).

[0064] A second integral part of DETS is the digital capture of thesignal. As depicted in FIG. 8b, the high, speed analog signal at timedelay D is captured directly into digital form. The receive trigger isused to open and close a fast switch or sample and hold, which feedsinto an A/D converter that outputs directly to a computer or otherdisplay device. The key point is that the transient data value at thegiven delay time is captured and stored as a contained action with noreference to other points which may be captured before or subsequentlyin time.

[0065] A DETS system can repeat the observation at a fixed delay anynumber of times and a computer or hard-wired logic circuit can take theindividual observed values from each repetition and average them toobtain an enhanced measurement with reduced noise.

[0066] The time delay, D, can be driven from a random number generator.In this case, the integer values m and n in FIG. 8 would be random andas a result the delay time would be randomized. If the observed data arerecorded along with the delay time associated with m and n then areconstruction of the waveform can be achieved by sorting the delaytimes in descending order and then plotting the observed signal versusdelay time.

[0067] With a DETS system, the time sampling can be discontinuous asshown in FIG. 4. In this case, there are two time windows recorded, onefrom time Δt to 3Δt and another one from 30Δt to 32Δt. Only three pointsare indicated here, but this could be generalized to any number ofpoints. Since the spacing between windows is programmable such anapproach can be used to develop an event tracker to record data from agiven delay time and ignore all others. For example

D₁=Δt

D ₂=2Δt

D ₃=3Δt

D ₄=30Δt

D ₅=31Δt

D ₆=32Δt  (4)

[0068] With DETS systems, one can acquire data in reverse order such asmight be obtained by the following sequence of delay times.

D ₁=6Δt

D ₂=5Δt

D ₃=4Δt

D ₄=3Δt

D ₆=1Δt  (5)

[0069] If there is a need to vary the stacking with delay time, then thedelay time can be fixed at a given value for a variable number ofrepetitions of the transmitter and signal averaged a variable number oftimes depending on time delay. A simple illustration of this is thefollowing table. Stat End Delay (6) 1  5 Δt${v({\Delta t})} = {\sum\limits_{1}^{5}{V_{i}/5}}$

6 10 2Δt${v\left( {2{\Delta t}} \right)} = {\sum\limits_{5}^{10}{V_{i}/5}}$

10  20 3Δt${v\left( {3{\Delta t}} \right)} = {\sum\limits_{10}^{20}{V_{i}/10}}$

[0070] Since the samples of a DETS system can be acquired at irregulartime intervals, the transmitter emissions can spread spectrum incharacter rather than spectral line in character as a regular repetitionof signal would entail.

[0071] By suitable design of the DETS system, the triggering paths andthe delays can be computer controlled and assembly of multi-channelsystems becomes practical. Such a DETS designed system provides apowerful multi channel capability. FIG. 9 shows the basic building blockof what is called a multi-channel DETS system (MDETS).

[0072]FIG. 9 shows the basic building block of a MDETS system. Computerscan enable this programmable time delay. The actual time delay takes aninput trigger either from an internal generated source or from anexternal source (i.e. a computer command etc.) which can be selectedunder computer control and responds to that trigger by generating atrigger output for a radar transmitter and a trigger output for a DETSsampling receiver.

[0073] The programmable delay allows coarse time steps in thetransmitter trigger and both coarse and fine delay steps in the receivertrigger so that the transmitter and receiver triggers can be offset withrespect to one another by fine delays and the whole operational unitdelayed by coarse steps. An output trigger is available from theprogrammed delay and this can be enabled or disabled by computercontrol. In addition to the delay system itself, the MDETS module canselect transmitter and receiver triggering from external sources as wellas internal sources. Since operation of all of these switches can beselected independently under computer control a very versatile buildingblock is developed.

[0074] The modular and compact nature of the timing and sampling withMDETS allows chaining of units in many ways. To allow all of thepossible forms of operation, MDETS modules are developed in two formsdenoted A and B, as shown in FIG. 10. The A unit provides fullversatility of input and output triggers and selection of operation. TheB form is a subset of A which has its main objective of acting as acontrol over an A-type unit, which acts as a slave. The B-type unit isprimarily required for managing synchronous operation of A type unitswhich are separated by substantial spatial distances.

[0075] In order to show how MDETS configurations allow implementation ofa variety of multi-channel operations, a standard schematic block isdepicted in FIG. 11. The type A response is a block with fourconnections on the top, two on the bottom and two internally generated.The unit is microprocessor controlled, has an embedded microprocessor aswell as a communications bus to allow it to interact with all of theother units that would be put in any multi channel system. The type Bblock is similar but has only a subset of the type A ports.

[0076] The simple modular schematics shown in FIG. 11 are used to showhow the interconnects for various operations can be managed. FIG. 12shows the range of interconnects from the simple to the complex. FIG.12a shows the simple individual single system as depicted in FIG. 10. Inother words, one MDETS type A unit will operate on its own and its onlyconnection to the outside world need be that of exporting data orimporting instructions as to what data it should collect.

[0077]FIG. 12b shows a dual unit system where one unit transmits and theother unit receives. This is a very common requirement in GPR and thespatial separation between the units can be highly variable and thistype of simple interconnect proves powerful. In this case we are stillreally using a single transmit/receive configuration.

[0078] The next mode of operation is that of handling multiple channelsof operation where time synchronization is not critical but interleavingoperation can be important. FIG. 12c shows how an arbitrary number ofunits can be set up to operate in this fashion. One B-type MDETS unit isused as a master control. This unit provides a synchronizing trigger toall of the active units.

[0079] Each of the individual units acts on its own and acquires datawhen commanded by the synchronizing trigger from the B MDETS unit. Allthe A units then function independently internally. The one factor whichallows interleaving operations is that all of the units can beprogrammed to carry out their data point acquisition at an arbitrarydelay after the common clocking trigger is received from the B unit. Asa result each unit can acquire data in a small time slot independent ofoperation of the other units. Obviously the time window whereoverlapping can occur will depend on the exact configuration of theradar but this can be programmed in to any level of resolution needed.

[0080] When we speak about interleave timing in such systems, all unitsonly have to have synchronized triggering to timing intervals on theorder of microseconds. On the other hand if one requires synchronoustime base acquisition within receivers then one may need timingresolutions to the order of tens to hundreds of pico seconds. Hence, wedistinguish between interleaved operation and synchronous operation fortiming requirements.

[0081] Fully synchronized time operation requires a B MDETS unit forevery A MDETS unit deployed. The concept is depicted in FIG. 12d. Theissue here is that the active A units are spatially distributed in anarray or a line and the distances between units can be quitesubstantial. As indicated previously, the travel time overinterconnecting cables can be significant. Delay times can be as big asthe transit time or the recording time of the signal. As a result it isimportant to be able to compensate for all of these time delaysassociated with by the spatial distribution so that all of the units canoperate precisely in a synchronous fashion.

[0082] The manner in which this is achieved is to have a B MDETS unitfor each A unit. The B units are all mounted in close proximity in asingle control unit with a master trigger to fire them allsimultaneously. Each B unit can be programmed to have an offset time,which accommodates all of the time delays associated with connections tothe individual A unit, which it controls. This timing can be controlleddown to the finest time resolution required for synchronous sampling forthe particular application.

[0083] The key point is that the B units are spatially close to oneanother in a self contained module and the A units are spatiallydeployed over an arbitrarily large area. Note that all units are timeprogrammable and operational programmable so that all of the necessarycorrection information can be learned and sustained and used within thesystem and interchanged digitally over the communications bus.

[0084]FIG. 13 describes a table outlining the various applications ordesired targets that the present invention may be applied to.

[0085] Various embodiments of the invention have now been described indetail. Since changes in and/or additions to the above-described bestmode may be made without departing from the nature, spirit or scope ofthe invention, the invention is not to be limited to said details.

We claim:
 1. For ground penetrating radar and the applications listed inFIG. 13, we claim the digital equivalent time sampling (DETS) andmulti-channel digital equivalent time sampling (MDETS) concepts asillustrated in the preceding discussion.
 2. The use of multi-channeldigital equivalent time sampling (MDETS) to coordinate two units, onefor transmitting and one for receiving in an arbitrary separationbetween the units for ground penetrating radar systems and for a desiredtarget.
 3. The use of MDETS for the operation of multiple units whichacquire interleave timing operations, but are commonly triggered from acentral source for ground penetrating radar and for a desired target. 4.The use of MDETS to run multiple distributed units which can bedistributed over a large spatial area but which will have synchronoustime operation of both transmitters and receivers for ground penetratingradar and for a desired target.
 5. The use of randomized triggering inall of the various modes so that the transmitter firing is irregular intime and can achieve noise suppression or interference suppression byspread spectrum operation for ground penetrating radar and for a desiredtarget.
 6. The use of multiple MDETS units to achieve higher equivalenttime sampling by feeding one receiver channel into several MDETS analogto digital (A/D) converter units which have sequence time delays and fora desired target.
 7. The use of MDETS to achieve a dynamic offset fortarget tracking such as might needed for a bottom tracker or a terraintracker where the recording window has to be offset to follow the eventin questions. With a digital, programmable time-base such adjustmentscan be made rapidly on the fly and also use the input of othertransducers which can provide distance information such as laser oraltimeters or laser range finders and for a desired target
 8. The use ofDETS and MDETS to achieve non-linear time sampling where the frequencycontext of the signal varies over the window of interest and for adesired target.
 9. A method of acquiring data of material usingtransducer means for ground penetrating radar comprising enhancing thedigital equivalent time sampling and multi-channel digital equivalenttime sampling.
 10. A method of transmitting and receivingground-penetrating signals wherein the transmitted signal is randomlytransmitted and then received so as to suppress interference signal. 11.A method as claimed in claim 10 wherein said signals are transmitted andreceived by DETS.
 12. A method as claimed in claim 11 wherein saidsignals are transmitted and received by multiple DETS.
 13. A device fortransmitting and receiving ground-penetrating signals comprising, atransmitter and a receiver wherein said transmitter randomly transmittedsaid transmitted signal.
 14. A device as claimed in claim 13 whereinsaid transmitter is a DETS means.
 15. A device as claimed in claim 13wherein said transmitter is a MDETS.